This invention relates to methods and apparatus for fabricating optical waveguide gratings.
Optical waveguide gratings, such as optical fibre gratings, can be formed by exposing the core of an optical fibre to an interference pattern defining the period of the grating. The first experimental demonstration of fibre grating formation in the core of an optical fibre was by launching laser light along an optical fibre from an Argon-ion laser operating at 514 nm, so that a grating was formed over the entire length of the fibre.
More recent techniques have used interference patterns incident on the side of the fibre to impress a grating structure on photosensitive regions within the fibre core and/or cladding.
The methods used to generate such an interference pattern have included prism-interferometers [see publication reference xe2x80x9cBroerxe2x80x9d cited below], diffraction grating/phase-masks [Anderson] and combined phase-mask and prism-interferometers [Armitage]. A further development is described in GB-A-2 272 075, where a phase mask is imaged onto the fibre core using a lens.
However, an established problem is that of tuning the pitch of the grating in the fabrication processxe2x80x94for example, to generate a long (several centimetres or more) chirped grating where the pitch varies along the length of the grating.
This is a very difficult task with an multi-beam interferometer, because the coherence length of the writing laser beam makes careful matching of the different optical paths critical to maintaining a good visibility of the generated interference pattern.
Tuning of the grating pitch from uniform phase masks have been reported before from both a phase mask magnification technique [Prohaska] and a tuning scheme based on a beam diameter dependant maximum tuning [Cole and GB9509874.5]. In both cases the maximum tuning possible isxcx9c(about) 10 nm (nanometres) with the latter system being the more flexible.
The present invention provides optical grating fabrication apparatus comprising
a phase mask for dividing an incident light beam into a plurality of diffracted beams; and
a focusing arrangement for receiving light from the phase mask and converging at least two non-zero-order diffracted beams together so as to generate an interference region between the converged beams so that a grating structure can be impressed on an optical waveguide placed in the interference region;
the phase mask and at least a part of the focusing arrangement being moveable with respect to one another so as to alter the angle of convergence of the converged beams.
In the invention, a phase mask is imaged onto the waveguide using a lens. However, rather than the usual step of fixing the relative positions of the phase mask and lens to a separation equal to the focal length of the lens (which would normally be expected for maximum mechanical stability of the system), the counter-intuitive step is. taken of altering the separation of the phase mask and lens to alter the angle of convergence of the conversed beams. The skilled man will immediately then understand that this in turn alters the fringe pitch incident on the waveguide.
Prototype embodiments of the invention can demonstrate a tuning range of 27 nm, i.e. about three times the best tuning range so far reported.
The advantages of the invention compared with other techniques used to write fibre Bragg gratings and in particular when used together with techniques where the fibre is moved in the interference pattern behind the either chirped or uniform phase- mask, e.g the xe2x80x98Step and repeatxe2x80x99 technique, by GB9617688.8, are that it provides no contact or close contact between the fibre and phase mask thereby avoiding the static electricity that builds up between moving glass surfaces.
By using a lens, the writing beam power can be focused more precisely onto the optical waveguide core of the grating host rather than through the side of e.g the optical fibre.
It has been known for two years before the priority date of this application [Ouellette] that the production of long phase-masks ( greater than 5 cm) is not possible with continuous techniques, and so step-write e-beam techniques must be employed. The repeat precision between concatenated sections, however, is not good enough to ensure a separation on the phase-mask period thereby introducing periodic xe2x80x98stich-errorsxe2x80x99 along the length of the phase-mask. Imperfect overlap regions will cause phase-shifts that will reduce the quality of the gratings written from such a phase-mask. Even scanning along non-stitched phase-mask will limit the grating quality to the uniformity in the groove depth pattern of the phase-mask. A variation in the groove depth of the phase-mask will cause a power fluctuation in the zeroth-order hence power fluctuations in the interfering xe2x88x921st and 1st orders of the phase-mask thereby leading to an increased background (dc) level in the grating and a reduction in the visibility of interference pattern. All these factors have tended to reduce the quality of the gratings produced. In contrast, the invention can avoid or alleviate this phase-mask grating quality degradation simply because the writing beam position on the phase-mask can be kept constant.
Although the invention can be embodied as a xe2x80x98free-spacexe2x80x99 interferometer (sometimes perceived as a disadvantage), the invention can in fact be embodied using only a single lens to catch and recombine the interfering beams. Furthermore, the tuning scheme is advantageously simplified because it only includes a relative movement of the lens with respect to the phase mask. If this movement is made along the direction of the writing beam, the coherence between the two interfering beams is not affected.
The invention also provides an optical grating fabrication method comprising the steps of: directing a light beam onto a phase mask to divide the light beam into a plurality of diffracted beams; converging at least two non-zero-order diffracted beams together using a focusing arrangement so as to generate an interference region between the converged beams so that a grating structure can be impressed on an optical waveguide placed in the interference region; and providing relative movement between the phase mask and at least a part of the focusing arrangement so as to alter the angle of convergence of the converged beams.